1. Field of the Invention
The present invention relates to a condenser with built-in deaerator and a method of starting and stopping the same. More particularly, the present invention relates to a condenser with built-in deaerator for use in a combined steam and gas turbine cycle and a method of starting and stopping the same.
2. Description of the Related Art
It is known to operate a power plant of a combined steam and gas turbine cycle system in a daily start-stop (DSS) mode. In DSS mode the power plant is started and stopped repeatedly according to variations in daily power demand. When operating the power plant in DSS mode, reducing the time necessary to start the power plant is an important concern. Various techniques have been developed to solve this problem. The time required for reducing the dissolved oxygen concentration of boiler feedwater in the steam turbine cycle to a reference value is one of the factors dominating the time necessary to start the power plant of a combined steam and gas turbine cycle system. Boiler feedwater, in most cases, is stored in a condenser. The condenser is opened to the atmosphere while the operation of the power plant is stopped, because it is more economical to open the condenser to the atmosphere than to maintain a vacuum in the condenser while the power plant is stopped. However, when the condenser is opened to the atmosphere, steam condensate (boiler feedwater) stored in the condenser is exposed to the atmosphere. Oxygen contained in the atmosphere dissolves in the condensate, and the dissolved oxygen concentration of the boiler feedwater increases nearly to the point where the dissolved oxygen is saturated in the boiler feedwater. If the boiler feedwater containing a large concentration of oxygen is fed to a boiler, the equipment of the power plant will be corroded by electrochemical reactions. Therefore, oxygen contained in the boiler feedwater is removed by a deaerator to reduce the oxygen dissolved in the boiler feedwater to the lowest possible extent before feeding the boiler feedwater to the boiler. Currently, large-capacity power plants require a reference dissolved oxygen concentration of 7 ppb or below.
The deaerator removes oxygen dissolved in boiler feedwater by a solubility nonequilibrium reaction using direct contact between the boiler feedwater and a gas other than oxygen. Steam is usually used as the gas other than oxygen for deaeration.
A previously proposed deaerating method for a power plant of a combined steam and gas turbine cycle system employs a deaerator installed in a condenser. For example, a condenser of the type shown in FIG. 5 is disclosed in JP-A No. 3-275903. As shown in FIG. 5, a condenser 100 has a main casing 100a defining a space isolated from the external environment, and a tube bank 101 installed in the space defined by the main casing 100a. A vacuum pump 102 is connected to the tube bank 101 by a pipe. The tube bank 101 communicates with a separate hot well 103 by a pipe 104 provided with a shutoff valve 105. A deaerator 106 is disposed on the hot well 103 to deaerate steam condensate stored in the hot well 103. A feed pump 107 is connected by a pipe to the bottom of the hot well 103 to feed the steam condensate stored in the hot well 103 to a boiler, i.e., an apparatus that demands the steam condensate. A pipe 108 is branched from a pipe extending from the discharge side of the feed pump 107 and connected to an upper part of the deaerator 106 to form a circuit for circulating the steam condensate stored in the hot well 103 through the deaerator 106. A pipe 109 for supplying steam to the deaerator 106 is connected to a lower part of the deaerator 106.
In the conventional condenser 100 of this construction, the hot well 103 can be completely disconnected from the tube bank 101 by the shutoff valve 105. When the power plant is stopped, the hot well 103, which stores most of the steam condensate is isolated from the tube bank 101. The bank 101 is opened to the atmosphere while the power plant is stopped, and a vacuum is maintained therein to keep the steam condensate at a low oxygen concentration so that the steam condensate can be quickly fed from the condenser 100 to the boiler upon resumption of the operation of the power plant.
The condenser disclosed in JP-A No. 5-79776 or 5-296007 has a feed pipe connected to a hot well, and a deaerator combined with a tube bank to circulate the water stored in the hot well through the tube bank for deaeration.
However, in the foregoing condensers, part of the water is open to the atmosphere even though the hot well is isolated and a vacuum is maintained therein. The water, open to the atmosphere, is exposed to the atmosphere, so that oxygen dissolves therein. Drains flow from units of the power plant into the condenser even when the power plant is stopped. The drain capacity is as large as several tons, which is about 20 to 30% of the normal capacity of the hot well. The drains can be deaerated only by mixing the water contained in the hot well with water from the drains and circulating the mixture of water through the deaerator and the hot well. It is not possible to discharge the drains outside the system because the system must then make up for the quantity of water discharged from the drains.
Problems concerning the deaerator built in the conventional condenser will be enumerated below.
In the condenser without a hot well capable of maintaining vacuum, oxygen continues to dissolve in the water contained in the condenser until the oxygen concentration in the water reaches approximately its solubility. In that case, a large quantity of steam and a long time is required to reduce the concentration of the oxygen dissolved in the water to the reference oxygen concentration.
The condenser with a hot well capable of holding a vacuum, and a deaerator that blows steam into the steam condensate remaining in a tube bank also has problems. In that condenser, if steam is blown into the steam condensate immediately after the resumption of the operation of the condenser, the steam will instantly condense because the pressure in the condenser is relatively high and the condenser will not be sufficiently evacuated immediately after the resumption of the operation of the condenser. Consequently, the steam condensate is merely heated and is barely deaerated. Since the bubbles barely stir the water only the portion of the water which is above a level corresponding to a position from which steam is blown into the water is heated. Also, the deaerator that disperses water in steam requires a large space. The dearator has little deaerating ability because the partial pressure of air (oxygen) in a gas space for steam is high immediately after the start of the vacuum pump.
The conventional deaerator effectively dearates only after the vacuum of the condenser has increased, the water has been superheated beyond the saturation temperature, and the steam condensate has begun flash evaporation. However, only part of the steam condensate in the high-temperature surface layer of the steam condensate flashes, while the remaining deeper portion of the steam condensate does not flash because the deeper portion is not heated and remains cold. Therefore, the deeper portion of the steam condensate does not flash and is not deaerated.